1100 Aluminum

Very soft & ductile means tendency to smear, pick up abrasive, show “orange peel” / relief. So use low loads, sharp SiC, short steps, good lubrication. Aim for minimal cold work: over-grinding makes grain boundaries hard to reveal, even with good etching. Prefer diamond + final colloidal silica or fine alumina for a flat, scratch-free surface.

Use abrasive cut-off with Al-appropriate cutting wheel (non-ferrous / soft metal wheel). Plenty of coolant; avoid overheating (no color change). Keep feed rate very light; 1100 will easily deform/burr at the cut edge. Avoid clamping that plastically deforms the piece; use broad, soft jaws or backing supports. Leave a small allowance (~1–2 mm) for grinding away the deformed surface from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the very soft material. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity. The mount should be slightly harder than the sample for better edge retention.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. The very soft material requires careful handling during mounting to avoid deformation. Ensure proper cooling under pressure to minimize shrinkage.

The extreme softness (23 HB) of 1100 Aluminum requires very careful grinding to avoid smearing and deformation. Use standard SiC grinding papers with abundant water lubrication. Disc speed: 200-300 RPM. Apply light pressure (15-25 N per 30 mm sample) - the very soft material is extremely prone to smearing if too much pressure is applied. Use sharp, fresh grinding papers to minimize deformation. Over-grinding can affect grain boundary revelation during etching.

Grinding sequence:

• 240 grit: Remove sectioning damage (20-40 seconds). Use light pressure to avoid deep scratches. Start at P240 (standard) or P320/P400 for minimal sectioning damage.
• 320 grit: Remove previous scratches (20-40 seconds). Monitor for smearing.
• 400 grit: Further refinement (20-40 seconds). Ensure complete scratch removal.
• 600 grit: Final grinding step (20-40 seconds). Prepare surface for polishing.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Abundant water lubrication is critical - avoid drying during grinding which can cause smearing. Grind just long enough to remove the previous scratch orientation - over-grinding can affect grain boundary revelation.

The extreme softness requires very careful polishing to avoid smearing and excessive relief. Use diamond polishing with appropriate polishing pads for each stage. Apply light pressure throughout to prevent deformation.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (15-20 N per 30 mm sample). Start with 6μm rather than 9μm to minimize damage to the very soft material. Monitor for smearing and reduce pressure if needed.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure. Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a soft synthetic pad with lighter pressure (10-15 N). These pads provide gentle material removal suitable for very soft aluminum.

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad (e.g., MICROPAD) with very light pressure. Soft pads are recommended for colloidal silica on soft materials and produce a mirror finish. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The very soft material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing, orange peel, or excessive relief.

1100 Aluminum responds well to standard aluminum etchants. The pure aluminum composition means it will etch clearly and show good grain structure. Three primary etchants are recommended:

Keller's Reagent (Chemical Etching) - Primary choice for general microstructure:

• Composition: 2ml HF, 3ml HCl, 5ml HNO₃, 190ml H₂O
• Preparation: Add acids to water slowly with stirring. Prepare fresh for best results. Store in plastic container (HF attacks glass).
• Application: Immerse sample or swab for 10-20 seconds. Classic general-purpose micro-etchant for Al alloys.
• Reveals: Grain boundaries and grain structure clearly. Excellent for general microstructure examination.
• Rinse: Immediately with water, then alcohol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several weeks if stored properly. Use in fume hood.

Weck's Reagent (Color/Tint Contrast) - For color/tint contrast or interfaces:

• Composition: Ammonium bifluoride 4.5g, HCl 10ml, Water 240ml
• Preparation: Dissolve ammonium bifluoride in water, then add HCl. Solution is stable for several weeks.
• Application: Immerse sample or swab for 15-30 seconds. Less common in routine work but great for photography and interface studies.
• Reveals: Color/tint contrast and interfaces. Excellent for photography and interface studies.
• Rinse: Immediately with water, then alcohol. Dry with compressed air.
• Note: Provides color contrast useful for photography. Use in fume hood.

0.5% HF (Light Etching) - For sensitive microstructures or when Keller's is too aggressive:

• Composition: 0.5ml HF (48%) in 100ml distilled water
• Preparation: Add HF to water slowly with stirring. Solution is stable for several months if stored in plastic container.
• Application: Immerse sample or swab for 15-30 seconds. Less aggressive than Keller's reagent, good for revealing fine grain structure without over-etching.
• Reveals: Grain boundaries and fine grain structure. Less contrast than Keller's but gentler on the microstructure.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Ensure good ventilation. HF is highly toxic - use proper PPE and fume hood.

Etching Strategy:

• Start with Keller's reagent for general microstructure examination - it provides good contrast and reveals grain boundaries clearly
• Use Weck's reagent for color/tint contrast or interface studies, especially for photography
• Use 0.5% HF if Keller's is too aggressive or for revealing fine grain details
• Always clean and degrease before etching
• Use short initial etch times (a few seconds), check under the microscope, repeat if needed
• Check etching progress frequently - over-etching can obscure fine details

Safety: All etchants contain HF or fluoride salts – full PPE (gloves, safety glasses, lab coat), proper fume hood, and HF-specific safety measures are mandatory. HF can cause severe burns and is toxic if ingested or absorbed through skin.